1. Field of Invention
This invention relates generally to a mechanism for manipulating and measuring limb movement, and in particular to a programmable, backdriveable device for locomotion training and assessment.
2. Related Art
Mechanisms for manipulating limbs and measuring limb movement have general utility in applications such as athletic training, orthopaedic rehabilitation, virtual reality, and scientific investigation. For patients who cannot walk as a result of traumatic spinal cord injury or stroke, devices that control and measure limb movement provide a means of precisely controlling locomotion training to take advantage of a relatively new rehabilitative technique called xe2x80x9cbody weight supported locomotion training.xe2x80x9dThis technique involves suspending a spinal cord injured patient in a harness above a treadmill and assisting the patient""s legs to move in a walking pattern. The underlying scientific basis for this new technique is the observation that after a complete thoracic spinal cord transection, the hindlimbs of cats can be trained to fully support their weight, rhythmically step in response to a moving treadmill, and adjust their walking speed to that of a treadmill (3,4,7). Results from several laboratories indicate that body weight supported training can also improve stepping in spinal cord injured humans, and that body weight supported training is superior to conventional rehabilitation (2,5).
Current body weight supported training techniques rely on manual assistance to the legs by several therapists to generate the swing phase of stepping and to stabilize the knee during stance. This manual assistance has several important scientific and functional limitations. First, the assistance can vary greatly between therapists and sessions. The patient""s ability to step on a treadmill is highly dependent upon the skill level of the persons conducting the training. Second, the therapists can only provide a crude estimate of the required force, torque and acceleration necessary for a prescribed and desired stepping performance. To date, all studies and evaluations of step training using body weight supported training over a treadmill have been limited by the inability to quantify the joint torques and kinematics of the lower limbs during training. This information seems critical to fully assess the changes and progress attributable to step training with the body weight supported training technique. Third, the manual method requires three or four physical therapists to assist the patient during each training session. This labor-intensive protocol is too costly and impractical for widespread clinical applications.
There is a need for a mechanism to assist neurally damaged patients to undergo body weight supported training. Such a mechanism can alleviate the deficiencies inherent in current manual assistance techniques.
An important issue in the design of such a mechanism for movement training is backdriveability, defined as low intrinsic endpoint mechanical impedance, or simply as the ability to move a device by pushing on its linkages. Good backdriveability has several advantages. It allows the patient to move freely when the actuators are not powered. Thus a backdriveable device could record movements of the patient in order to quantify recovery progress. Backdriveable machines can also be made to xe2x80x9cfade to nothingxe2x80x9d by reducing the amount of assistance they provide as patient recovery improves. Additionally, a backdriveable device can be controlled in such a way that it deviates from a controlled path when the patient exerts uncoordinated forces, providing direct and natural kinematic feedback of movement control errors. In contrast, a non-backdriveable device must rely on force sensing and visual, tactile, or auditory feedback of the sensed force to provide feedback of movement error. A possible safety advantage is that an impedance-controlled, backdriveable machine can xe2x80x9cget out of the wayxe2x80x9d of the patient if the patient rapidly changes his or her pattern of force development. Finally, a backdriveable machine can be designed to record movements and forces applied by therapists, then replay them.
In addition to backdriveability, a mechanism for movement training can benefit from the incorporation of robotic technology. Robotics provides a way to precisely control leg movement during treadmill training and to quantify in real time the kinematics and kinetics of stepping. The application of robotics to locomotion training could ultimately lead to automated treadmill training and monitoring in the clinic, reducing the cost of training and increasing accessibility.
Progress in developing robots for locomotor therapy is being made. The Mechanized Gait Trainer is a singly-actuated mechanism that drives the feet through a gait-like trajectory (6). The device consists of two foot plates connected to a doubled crank and rocker system. An induction motor drives the cranks via a planetary gear system. The rear ends of the foot plates follow an ellipsoid-like movement. Different gears can be incorporated to vary stride length and timing. The planetary gear system also moves the patient harness in a locomotion-like trajectory through two cranks attached to suspension ropes. The torque generated by the motor is sensed and displayed on-line to provide a biofeedback signal to the patient. The device has been used to train two patients who were two months post-stroke. The patients received four weeks of gait training with the device, consisting of five 20-minute sessions per week. The patients improved significantly in their overground walking ability.
The Lokomat is a motorized exoskeleton worn by the patients during treadmill walking (1). This device has four rotary joints that accommodate hip and knee flexion/extension for each leg. The joints are driven by precision ball screws connected to DC motors. Parameters such as the hip width, thigh length, and shank length can be manually adjusted to fit individual patients. The weight of the exoskeleton is supported by a parallelogram mechanism that moves in the vertical direction and is counterbalanced by a gas spring. The hip and knee motors can be programmed to drive the legs along gait-like trajectories. The device is relatively stiff and is difficult for the patient to move under his own power. Therapeutic results have not been reported for the Lokomat, although several spinal cord injured patients have tested the device. The device was able to drive gait-like patterns in the patients, reducing the labor burden on the therapists who were assisting in the step training.
While cleverly designed and useful, these two robotic devices lack backdriveability. The Mechanized Gait Trainer is not fully backdriveable because it cannot be driven away from the path specified by its single degree-of-freedom mechanical linkage. The Lokomat is difficult to backdrive because it uses high-advantage, ball-screw actuators. Backdriveability with substantial actuator power is in general difficult to achieve, although some backdriveability can be endowed to a non-backdriveable device by sensing the contact force between the device and the environment, and moving the actuators in order to control that force. The simplest and most robust approach to good backdriveability, however, remains the minimization of friction and inertia of the mechanism and actuators.
It is an object of the present invention to incorporate a backdriveable robotic device into locomotion training and assessment The device should be capable of generating substantial forces for assisting in stepping, while minimally encumbering the legs of a patient.
Accordingly, the present invention provides a backdriveable device for measuring and manipulating limb movement. The device comprises a plurality of forcers, at least one linear guide for directing the forcers to move back and forth along their respective linear paths, and a linkage rotatably connected between the forcers and including two bars rotatably connected together to form a vertex. As the forcers move along their linear paths, they apply force to the linkage, resulting in movement of the vertex in a spatial trajectory. For locomotion training, the linkage can be connected at the vertex to a patient""s leg by attachment to the lower shank, pelvis, knee, ankle or sole of the foot.
In particular embodiments, the forcers are moving coils of a linear motor. Mounted on a linear bearing, the coils provide substantial force with minimal friction. The movement and forces applied by the coils can be controlled through a computer interface, providing robotic control of limb movement.
The present invention also provides a method of measuring and manipulating limb movement using a backdriveable device. The method comprises: (a) providing a plurality of forcers moving along respective linear paths; (b) rotatably connecting a linkage between the forcersxe2x80x94the linkage includes two bars rotatably connected together at a vertex, and the forcers and linkage are connected such that movement of the forcers along their respective linear trajectories results in movement of the vertex in a spatial trajectory; (c) attaching the linkage to a limb of a user; and (d) moving the plurality of forcers along their respective paths. In practice, movement of the forcers can initiate movement of the limb along a spatial trajectory, or the limb can first move along a spatial trajectory, driving the passively moving forcers along their linear paths.
The backdriveable device of this invention has a number of advantages. For example, when practiced with a moving coil linear motor that generates substantial force with low backdrive friction, the device is powerful yet lightweight. Also, when applied to locomotion training, force application can be mechanically constrained to a physical workspace that matches that of the leg during walking (i.e. horizontally wide but vertically short) with simple hard stops or by installing short links. Another advantage is that in contrast to exoskeletal approaches, the device can accommodate any size leg and any stepping trajectory without mechanical adjustment.
The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.